Semiconductor devices have become highly integrated and are operated at a high speed. Accordingly, the size of a transistor has become gradually reduced. As the integration degree of a transistor increases, a metal line of a semiconductor device is fabricated in a micro size. As a result, signals applied to the metal line are delayed or distorted due to parasitic resistance and capacitance and thus the high-speed operation of the semiconductor device is interrupted.
In order to solve such a problem, a copper line using copper has been rapidly developed because the copper line has lower resistance and higher electro-migration as compared to aluminum or aluminum alloy that was widely used as metal line material of a semiconductor device.
In order to form a copper line, a copper layer is formed and then must be etched. However, copper is not as easily etched as aluminum, and the surface of a copper layer is rapidly oxidized in the air. Accordingly, a damascene process has been developed in order to solve such a problem when forming a copper line.
According to the damascene process, via holes and trenches are formed in an interlayer dielectric layer, a copper layer is deposited on the interlayer dielectric layer such that the via holes and trenches are filled with the copper layer, and the copper layer is planarized through a Chemical Mechanical Polishing (CMP) process, so that copper lines are formed in the via holes and trenches.
Copper ions contained in the copper lines are easily diffused to the interlayer dielectric layer and may cause a short circuit with respect to adjacent metal lines. Accordingly, via holes and trenches are formed in the interlayer dielectric layer, and then barrier layers are formed on the inner walls of the via holes and trenches in order to prevent copper ions from being diffused.
However, since the barrier layers formed on the inner walls of the via holes and trenches have an inferior step coverage, undesired voids may be produced when the copper lines are formed on the inner walls of the via holes and trenches. In particular, voids may form due to the sputtering process of a thin barrier metal layer.